Transverse variations of sound pressure in an ear canal occluded by a hearing aid

Abstract

The sound field in a model ear canal has been investigated experimentally and theoretically. The canal was cylindrical, 7.5-mm diameter, and terminated with a Zwislocki coupler to represent absorption by the human middle ear. The outer end of the canal was driven by a test hearing aid, with the acoustic output from a receiver entering the canal through a 1-mm port. The hearing aid was provided with a 20-mm-long vent, either 1 or 2 mm in diameter. The sound field inside the canal was measured using a specially designed 0.2-mm-diam probe microphone [Daigle and Stinson, J. Acoust. Soc. Am. 116, 2618 (2004)]. In parallel, calculations of the interior sound field were performed using a boundary element technique. Both calculations and experiment reveal large transverse variations of sound pressure level, as much as 25 dB at 8 kHz, near the inner face of the hearing aid. Variations are greatest near the outlet port of the receiver and the vent hole. Deeper into the canal, the transverse variations are less significant and, at depths greater than 5 mm, only a longitudinal variation remains. [Work supported by Gennum Corp.]